The present invention relates to an adaptive filter for echo cancellation, methods for operating an adaptive filter for echo cancellation, an article of manufacture for determining tap weights and a length for an adaptive filter for echo cancellation and a computer implemented control system for determining tap weights and a length for an adaptive filter for echo cancellation.
FIG. 1 is a simplified schematic diagram of a system 10 for coupling communications signals 12 from a first point 14 to a second point 16 via a transmission medium 18 and 18xe2x80x2 and vice versa, in accordance with the prior art.
Impedance changes or discontinuities associated with the receiving equipment at the second point 16 can cause a reflected signal or echo 22 to be sent back to the first point 14 when the signal 12 from the first point impinges on the second point.
The reflected signals 22 may be characterized by a magnitude, or attenuation, relative to the communications signals 12, a delay relative to the communications signals 12 and dispersion of reflected signals 22 in time.
It is generally desirable to remove the reflected signals 22 from the transmission medium 18xe2x80x2 order to be able to optimize intelligibility for the communications signals at the first point 14. However, in systems such as telephone systems, the signal transmission path through the transmission medium 18, 18xe2x80x2 to the second point 16 may change with each call. For example, there may be more than one communications station that is able to respond as the second point 16. As a result, the reflected signals 22 may be different for each call.
Moreover, in modern communications systems, properties of the transmission medium 18, 18xe2x80x2 are also a function of time, with the result that the reflected signals 22 change relative to the signal 12 giving rise to the reflected signal 22 with time during a single call. An effective echo cancellation apparatus must be able to alter the echo cancellation in response to changes in the properties of the transmission medium 18 and/or second point 16 during a given data exchange or communications session. An example of a typical telephone system showing digital and analog portions thereof is given in U.S. Pat. No. 5,793,864, entitled xe2x80x9cNonintrusive Measurement Of Echo Power And Echo Path Delay Present On A Transmission Pathxe2x80x9d, issued to Ramsden and which is incorporated herein by reference for its teachings.
Many different schemes have been devised for trying to compensate for these effects in real time. An example is described in U.S. Pat. No. 5,289,539, entitled xe2x80x9cEcho Canceller Using Impulse Response Estimating Methodxe2x80x9d, issued to Y. Maruyama and hereby incorporated herein by reference for its teachings on this topic. The method described in U.S. Pat. No. 5,289,539 relies on least mean square (LMS) calculations to set tap weights for a finite impulse response (FIR) filter, based on measured properties of signal reflections. As noted in this patent, these calculations can be impracticable when excessive numbers of arithmetic operations are required, and there are tradeoffs between rapidity of convergence for such calculations and the degree of success in cancellation of the reflected signals 22.
Many of these calculations may be unnecessary, because the reflected signals 22 tend to result from only a few points associated with the second point 16, while the calculations pertain to more of the transmission medium 18 coupled to the second point 16. As a result, many of the computations that are taking place tend to be operations carried out on nullities insofar as the final result is concerned.
Echo cancellation parameters need to be updated frequently enough to reflect the actual signal reflection conditions accurately, requiring repetition of the calculations. Accordingly, reduction in the number of calculations needed for effective echo cancellation is desirable.
What is needed is a method and a corresponding apparatus for allowing reflected signals in signal transmission systems to be canceled without incurring excessive computations.
In one aspect, the present invention provides an adaptive filter for echo cancellation. The adaptive filter includes a segmented sparse transversal filter having an input, the filter having an adjustable length, adjustable lengths for the segments and adjustable tap weights. The adaptive filter further includes an adaptive tap weight control mechanism providing a tap weight vector including tap weights and a tap weight vector length to the taps of the transversal filter, the adaptive tap weight control mechanism setting the tap weights and the tap weight vector length in response to comparison of integrated cross correlation coefficients between a reference signal and an error signal from the adaptive filter.
In another aspect, the present invention includes a method for operating an adaptive filter for echo cancellation. The method includes generating estimated truncation errors of a tap weight vector for a sparse segmented transversal filter, calculating integrated cross correlation coefficients between a reference signal and an error signal from the sparse segmented transversal filter and comparing the estimated truncation errors to a target truncation error. The method also includes setting a length of the tap weight vector in response to comparing the estimated truncation errors to the target truncation error.
In a further aspect, the present invention includes a method for operating an adaptive filter for echo cancellation. The method includes determining a length for the sparse segmented transversal filter from the estimated reflective properties and adapting a set of tap weights for each segment of the sparse segmented transversal filter from the estimated reflective properties.
In yet another aspect, the present invention includes an article of manufacture comprising a computer usable medium having computer readable code embodied therein to cause a processor to determine a length for the sparse segmented transversal filter from the estimated reflective properties and to adapt a set of tap weights for each segment of the sparse segmented transversal filter from the estimated reflective properties.
In a still further aspect, the present invention includes a computer implemented control system for determining tap weights and a length for an adaptive filter for echo cancellation. The system includes memory configured to store tap weights and processing circuitry. The processing circuitry is configured to determine a length for the sparse segmented transversal filter from the estimated reflective properties and to adapt a set of tap weights for each segment of the sparse segmented transversal filter from the estimated reflective properties.